In a wireless communication system, a wireless terminal, e.g., mobile node, is often coupled to a wired network, such as the Internet, via base stations. Base stations provide network connectivity within a coverage area called a cell. The communications path from a base station to the wireless terminal is called a “downlink”, while a communications path from the wireless terminal to a base station is called an “uplink”.
For the purpose of network connectivity, at a minimum, the wireless terminal communicates with one base station. However, for various performance considerations typically in support of terminal mobility, e.g., moving from cell to cell, wireless terminals are often equipped to simultaneously maintain wireless link connections with multiple base stations. For example, in a Code Division Multiple Access (CDMA) system, the wireless terminal can be in a “soft handoff” state.
FIG. 1 illustrates a known CMDA communications network 100, which includes wireless terminal 102, base station 1 104, base station 2 106, mobile switching center 108 and wired networks 110. Various communications between the network elements are represented by arrows. Uplink communications are illustrated in FIG. 1 and downlink communications are illustrated in FIG. 2.
The existing method of soft handoff in direct spread CDMA technologies delivers a single data information flow split across multiple link connections, each from a different base station, to a wireless terminal, and another data flow from the terminal back to the multiple base stations. The consequent characteristics, including tight time synchronization of these connections between the base stations and the terminal, constrains the technology choice for use in a radio access network infrastructure.
Referring to FIG. 1, in the soft handoff state, using the uplink, the wireless terminal 102 transmits a signal 112, 114 representing information to be sent. In this example, the wireless terminal 102 is in the coverage area of more than one base station 104, 106. Therefore, more than one base station 104, 106 listens to the same uplink signal 112, 114 simultaneously.
Then, in response to receiving the signal 112, 114 from the wireless terminal 102, the base stations 104, 106 process received signals. As represented by arrows 116 and 118, the processing results are sent to a central unit, often referred to as a mobile switching center 108, which combines the results from individual base stations 104, 106 to obtain the sent information. Then the mobile switching center 108 sends the information to the wired network 110, e.g., the Internet. This is represented by arrow 120.
Similarly, in regard to the downlink, as illustrated in FIG. 2, the mobile switching center 108 receives information from the wired networks 110 for the wireless terminal 102, as represented by arrow 220. Then the mobile switching center 108 duplicates the information and transmits the information to more than one base station 104, 106. This is represented by arrows 216, 218. The base stations 104, 106 simultaneously transmit the received signal representing the information to the wireless terminal 102. This is shown using arrows 212 and 214. The wireless terminal 102 combines the signals received from the base stations 104, 106 to obtain the information from the wired networks 110.
One advantage of having the soft handoff state is to achieve macro diversity. In addition, the soft handoff state also reduces data loss and latency during handoff, i.e., when the wireless terminal is switched from one base station to another data is transmitted by multiple base stations.
Data received by a wireless terminal from a first base station can be combined with data received from a second wireless terminal which transmits the same signal to form a complete message or set of data even when communication with the first base station is lost, e.g., due to entry into the coverage area of the second base station.
Soft handoff has the disadvantage associated with the complexity and timing requirements of utilizing mobile switching center 108 as a combining unit in the uplink and as a duplicating unit in the downlink. This characteristic constrains network operations, since this characteristic requires synchronized network transport technology capable of delivering data information to and from the mobile switching center 108 and the base stations 104, 106 with very low delay jitter with respect to the multiple base stations 104, 106. That is, in such systems information to and from the multiple base stations 104, 106 and the mobile switching center 108 must be tightly synchronized in time. This synchronized-network transport characteristic stands in marked contrast to the operation of packet-switched data networks that typically utilize asynchronously-networked transport technologies.
In a mobile communication system, maintaining multiple link connections simultaneously is important to ensure seamless handoffs. However, there are compelling economic advantages in using more asynchronous forms of data networking technologies within radio access networks. Accordingly, there is a need for improved methods and apparatus for enabling wireless connectivity, at least some of which will allow the wireless terminals to be simultaneously connected with multiple base stations while enabling the base stations to communicate with the wired network in a way consistent with asynchronous, packet-switched data networking.